The image reading apparatus is generally composed of an illumination unit, an image forming system such as a lens, and light-electricity transducing elements. The composing elements required for reading an image are substantially the same in the image scanner, the digital copying machine, and the facsimile, therefore these composing elements are described by taking the case of an image scanner, by way of example.
In the ordinary image reading apparatus, a linear light source such as a mercury fluorescent lamp or halogen lamp is used as the illumination unit; a lens of about (7 against 1) reduction ratio is used as the image forming system; and linearly arranged CCDs are used as the light-electricity transducing elements. An example of the prior art image reading apparatus is disclosed in Japanese Published Unexamined Patent Application No. 60-148269. FIG. 9 is a cross-sectional view showing this prior art image reading apparatus, in which the light reflected from an image 20 illuminated by an illumination unit 22 is image formed on the CCDs 25 of the light-electricity transducing elements through a mirror 23 and a lens 24 which constitute an image forming system. In the image forming system as described above, in order to increase the image reading speed, the following four methods have been so far adopted: (1) the sensitivity of the light-electricity transducing elements 25 has been increased; (2) a bright lens has been used by increasing the aperture ratio (F-number) of the image forming system 24; (3) the quantity of light from the illumination unit 22 has been increased; and (4) the quantity of light applied onto the light-electricity transducing elements 25 has been decreased to increase the reading speed, with decreasing the S/N ratio; that is, with deteriorating the image quality. Conventionally, the reading speed has been so far improved mainly by increasing the quantity of light emitted from the illumination unit 22 as described in item (3) above.
However, when the image reading speed is required to be further improved to such an extent that 20 pieces of A4-sized paper image can be read per minute, for instance such as in the case of an analog copying machine, if the quantity of light from the illumination unit is simply increased, the size of the image reading apparatus is inevitably increased. This is because a heat radiation plate or an exhaust heat fan are additionally required to exhaust heat generated by the illumination unit and further an additional air flow path will be necessary to exhaust the heat.
In the prior art optical system, the utilization efficiency of light emitted from the illumination unit is very low. Here, "illumination efficiency" can be defined by taking a ratio of the quantity of light for illuminating an object to be image read to the quantity of light for illuminating an area on the object to actually read the image. FIG. 10 shows a distribution of the quantity of light for illuminating the object 20 to be image read. As shown in FIG. 10, the ordinary illumination unit of the image reading apparatus illuminates a width of about 10 to 30 mm in the secondary scanning direction on the object 20 to be image read. Here, when taking into account the case where the reading width is 300 dpi, the light is applied upon a width of 30 mm at the maximum in order to read an image of an object 20 in about 85 .mu.m width. That is, the illumination efficiency is 0.85% to 0.28% in accordance with the following equation: ##EQU1##
In other words, the light of 99.15 to 99.72% is not only wasted but also needlessly illuminates the area other than is necessary to read the image, thus causing the generation of stray light. As a result, there arises a problem in that the quality of gradient in the read image is deteriorated markedly.
The reason why the illumination width as wide as 10 to 30 mm is secured in the prior art image reading apparatus is to keep the quantity of illuminating light at a constant level at the image reading position even if the object 20 is somewhat floated upward from the base position. FIG. 11 is an illustration for assistance in explaining the above-mentioned reason, which is an enlarged view showing the image reading position shown in FIG. 9. The central ray of light La for illuminating an object 20 is allowed to be incident upon the object 20 at an angle of 45 degrees, for instance. Here, assuming that the object 20 is floated up by a distance .DELTA.h from a glass base 21, the position P1 at which the central ray of light La emitted from the illumination unit 22 illuminates the object 20 is shifted to the position P2. In this case, however, since the optical image reading system is located on the optical axis Lb, the position at which the object 20 to be image read is still kept at the position P1'. In other words, there exists an offset .DELTA.y between the central position P2 of the illumination light and the image reading position P1'.
Consequently, in order to read the object 20 floated upward away by 20 mm from the glass base 21, for instance, it is necessary to secure the width of the illumination light to such an extent as to correspond to the above-mentioned distance of the object 20 from the glass base 21 in the secondary scanning direction (in a term of the image reading apparatus) or in the arrow direction (shown at the center of FIG. 11). The illumination efficiency is deteriorated by the reason as described above.